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Verified Computations for Closed Hyperbolic 3-Manifolds

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Verified Computations for Closed Hyperbolic 3-Manifolds Verified computations for closed hyperbolic 3-manifolds Matthias Goerner Abstract. Extending methods first used by Casson, we show how to verify a hyper- bolic structure on a finite triangulation of a closed 3-manifold using interval arith- metic methods. A key ingredient is a new theoretical result (akin to a theorem by Neumann-Zagier and Moser for ideal triangulations upon which HIKMOT is based) showing that there is a redundancy among the edge equations if the edges avoid “gim- bal lock”. We successfully test the algorithm on known examples such as the ori- entable closed manifolds in the Hodgson-Weeks census and the bundle census by Bell. We also tackle a previously unsolved problem and determine all knots and links with up to 14 crossings that have a hyperbolic branched double cover. Contents 1 Introduction2 2 Hyperbolic structures on finite triangulations4 arXiv:1904.12095v2 [math.GT] 6 Jan 2021 3 Cocycles 6 4 Extending cocycles on genus 0 surfaces9 5 Extending cocycles on triangulations 12 6 Examples of (non-)gimbal lock 14 7 Algorithm 15 2 8 Results 20 9 Discussion 21 10 Appendix: The uncited theorem HIKMOT relies on 25 1 Introduction Up to isometry, a finite hyperbolic 3-simplex is determined by its 6 edge parameters, by which we mean either the edge lengths li j or the respective entries of its vertex Gram matrix v cosh(l ). Thus, an assignment of a parameter to each edge of a finite i j Æ¡ i j triangulation T of a closed 3-manifold determines a hyperbolic structure for each 3- simplex of T . If certain conditions are fulfilled, the hyperbolic structures on the indi- vidual simplices are compatible and form a hyperbolic structure on the manifold (see [Hea05] and Section2). Existing software (such as Casson’s Geo [Cas] and Heard’s Orb [Hea]) finds a numerical approximation for the edge parameters using Newton’s method and reports whether the necessary equations are fulfilled within an error smaller than a certain ". This suggests but does not prove hyperbolicity. The aim of this paper is to describe how to take such a numerical approximation and rigorously prove hyperbolicity by giving real intervals that are verified to contain a solution to all the necessary equations and inequalities. An algorithm either returning such intervals or (conservatively) reporting failure is de- scribed in Section7. The algorithm is a hyperbolicity verification procedure but not a hyperbolicity decision procedure since its failure just means that the given candidate approximation was not close enough to a hyperbolic structure or needs to be perturbed to avoid “gimbal lock” (explained below). An implementation of this algorithm is avail- able at [Goe19b]. Therefore, this paper is achieving for finite triangulations what Hoff- man, Ichihara, Kashiwagi, Masai, Oishi, and Takayasu [HIKÅ16] did for ideal triangula- tions (HIKMOT’s functionality has been integrated into SnapPy [CDGW] by the author since version 2.3). This is motivated by applications that benefit from using geometric finite triangulations in place of geometric spun triangulation. In particular, such applications no longer need to overcome the incompleteness locus. An example is the generation of cohomol- ogy fractals for a closed hyperbolic 3-manifold. As shown in [BGSS20, Figure 8.7], the incompleteness locus produces artifacts in the raytraced image of a cohomology frac- tal which simply disappear when using finite triangulations instead. Another example is the algorithm proposed in [HHGT17] to rigorously compute the length spectrum for a hyperbolic 3-manifold. This algorithm requires tiling H3 with translates of a funda- mental domain to cover a ball of specified radius and thus would not work if there is incompleteness locus. An even more basic motivation is proving hyperbolicity of a closed 3-manifold by find- ing a triangulation admitting a geometric structure. Restricting ourselves to just spun 3 triangulations introduces a bottleneck. For example, the obvious spun triangulation of a closed census manifold such as m135(1,3) can fail to be geometric. Thus finding a geometric spun triangulation requires drilling and filling (or, in other words, finding a different closed geodesic γ such that there is a geometric triangulation spun about γ). Even worse, some hyperbolic 3-manifolds such as m007(3,1) seem to lack any geomet- ric spun triangulation unless we pass to a cover1. Note that a geometric spun trian- gulation is known for every orientable closed manifold in the SnapPy census (except for m007(3,1) where a 3-fold cover is needed), see [HIKÅ16]. However, the process of finding such (covers admitting) geometric spun triangulations can be tedious and is not known to be possible in general. Furthermore, passing to a cover can complicate appli- cations such as computing the length spectrum. Potential future work might generalize the techniques of this paper to Heard’s work [Hea05] on 3-orbifolds and Frigerio and Petronio’s work [FP04] on 3-manifolds with geodesic boundary. To find hyperbolic structures on these, Heard’s program Orb [Hea] uses triangulations with finite as well as ideal and “hyperinfinite” vertices. Note that some of the theory in this paper also carries over to spherical and Euclidean geometry and might generalize to yield methods for verifying spherical or Euclidean structures on finite triangulations. Like [HIKÅ16], we use interval arithmetic methods such as the interval Newton method or the Krawczyk test. These methods can only show the existence of a solution to a system of equations if the Jacobian matrix is invertible near that solution. If the Jacobian fails to be invertible, these methods can only show the existence of a solution to a subset of the equations. This applies to the edge equations whether we are solving for shapes in the cusped case or for edge lengths in the closed case. Hence, in both cases, we need an additional result showing that there is a redundancy among the edge equations such that solving a suitable subset of them is sufficient. For ideal triangulations, this result is due to Neumann-Zagier [NZ85, Neu92] and Moser [Mos09] (see Appendix). For finite triangulations, we derive such a result in this paper. Note that while we actually have exactly as many variables as equations in the case of finite triangulations (namely, one per edge), the Jacobian of this system of equations has a kernel at a solution corresponding to a hyperbolic structure. This is because we can move each individual finite vertex of a triangulation in the hyperbolic manifold and obtain a whole family of solutions (see Theorem 9.4). Thus, we need to use a two-step strategy to verify a hyperbolic structure: First, we drop some edge equations and fix an equal number of edge parameters such that we can apply interval arithmetic methods to find intervals verified to contain a solution to the subsystem of equations we kept. Next, we show that this solution is also a solution to the equations we dropped earlier and thus that the intervals for the edge parameters contain a point giving a hyperbolic structure. Interval arithmetic can verify that the error of the dropped equations is small and we will show that if the dropped equations are fulfilled approximately, then they are fulfilled exactly provided that a certain condition we call “gimbal lock” is avoided. 1In not yet published work, Maria Trnkova has proven that there is no geometric spun triangulation of m007(3,1) with a small number of tetrahedra. 4 To define gimbal lock, we will look at the complex of doubly-truncated simplices asso- ciated to the triangulation and an assignment of PGL2(C)-matrices to the edges of the complex computed from the edge parameters (see Section3). The cocycle condition says that the matrices on the edges of a polygon must multiply to the identity. Since a subset of the edge equations is known to be fulfilled, the cocycle condition is known to hold for some polygons but not necessarily for others. The goal is to show that it holds for all polygons so that we get a PGL2(C)-representation of the fundamental group (see Section4 and5 and examples in Section6). Roughly speaking, the idea is that if the product of three small rotations about three axes in generic position is the identity, then each rotation must be the identity. Inspired by the mechanical device called gimbal (see Figure8), we say that we avoid “gimbal lock”: if a gimbal is not in its locked posi- tion, then we can apply any small rotation to the inner-most ring, or equivalently, if we fix the inner-most ring, none of the other rings can be turned. We describe the resulting algorithm to obtain real intervals in Section7. The algorithm is effective and able to verify a hyperbolic structure on all 36093 closed orientable man- ifolds in the Hodgson-Weeks census [HW94] and in the census bundle by Bell [Bel15], see Section8. Branched double covers of knots or links (or more precisely: double cov- ers of S3 branched over a knot or link) provide a good class of test cases since finding a geometric spun-triangulation of some of them can be challenging. Using finite triangu- lations instead, we are able to prove the following new result: Theorem 1.1. Out of the 313230 knots with up to 15 crossings (not including the un- knot), exactly 193839 have a hyperbolic branched double cover. Out of the 120573 links with up to 14 crossings (with at least two components), exactly 37709 have a hyperbolic branched double cover. Section9 concludes with a conjecture that implies that a hyperbolic structure on a finite triangulation can always be perturbed so that the algorithm can verify it.
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